The background of the invention will be set forth in two parts.
1. Field of the Invention
This invention relates to thermoelectric devices and more particularly to stacked or staged thermoelectric modules capable of operation with large temperature gradients.
2. Description of the Prior Art
Devices capable of heating, cooling and power generation using thermoelectric principles and particularly the Peltier effect are well known in the art. These devices are generally referred to as thermoelectric coolers or generators and usually consist of thermoelectric couples connected in series and grouped into a device called a thermoelectric module. A more detailed description of thermoelectric modules may be had by making reference to many available publications on the subject such as, for example "Semiconductor Thermoelements and Thermoelectric Cooling" by A. F. Ioffe, published by Inforsearch Limited, London, 1957; and "Thermoelectric Materials and Devices," by I. B. Cadoff and E. Miller, Reinhold Publishing Corporation, New York 1960.
A thermoelectric module is capable of providing about 40.degree. C difference between its two opposite parallel planar surfaces, which difference is generally referred to as a temperature gradient for .DELTA.T for the device. Where greater temperature gradients are required, these modules are stacked one on the other to a maximum, with reliability, of usually only a three stage unit. Typically, this staged arrangement could obtain .DELTA.T's of from 50.degree. to 70.degree. C. In order to obtain greater temperature gradients, more stages are needed, However, these devices have very low expansion coefficients and are very brittle. It has been found that when exposed to high temperature gradients, these units will generally crack or strain solder joints which electrically bond the thermoelectric couples to their associated thermoelectric conducting bus bars. This has been the primary limitation on thermoelectric devices, and as a result, high temperature gradients have not been reliably obtainable in conventional thermoelectric coolers. It should therefore be evident that a thermoelectric system that provides a greater temperature gradient without the danger of damaging the thermoelectric modules would consititute a significant advancement of the art.